The present invention relates to measurement of wavelength dispersion distribution of an optical fiber, and particularly to a new measuring method and measuring apparatus useful for measurement of wavelength dispersion distribution of a long optical fiber.
In an ultra high-speed optical communication field at present, in order to implement and maintain high quality of communication, various researches have been advanced with respect to communication quality control and compensation technique of an optical fiber line.
With that, the need for characteristic evaluations of an optical fiber from the market is also increasing more than ever.
Among them, wavelength dispersion distribution characteristics of the optical fiber have been noted as an important item for evaluating limits of a transmission speed and a wavelength band.
Then, as an example of measurement of wavelength dispersion distribution of the optical fiber of this kind, an example described in JP-A-10-83006 is known.
A principle of measurement of wavelength dispersion distribution of the optical fiber will be described using a diagram showing a conventional configuration of a wavelength dispersion distribution measuring apparatus of an optical fiber of FIG. 2.
In FIG. 2, numeral 1 is a laser source 1 (LS1) for generating coherent light of a wavelength xcex1, and numeral 2 is a laser source 2 (LS2) for generating coherent light of a wavelength xcexc2, and the two light is combined by a coupler 3.
The light combined by the coupler 3 is shaped in pulse shape in synchronization with a clock signal (not shown) by an AO switch 4 and is amplified by an erbium doped fiber amplifier (EDFA) 5.
The light amplified by the erbium doped fiber amplifier 5 is emitted to an optical fiber 7 which is a measurement target by an optical circulator 6.
Also, the optical circulator 6 branches total back-scattered light generated by light launched into the optical fiber 7 and passes only light of a particular wavelength component through an optical band pass filter 9 to give it to an OTDR (optical time domain reflectometer) 10.
A terminator 8 suppresses Fresnel reflection in the far end of the optical fiber 7.
Incidentally, the optical band pass filter performs a function of extracting a wavelength component of one of four-wave mixed light generated by interaction of two wavelengths launched among the total back-scattered light by a light pulse signal inputted to a measured optical fiber.
In the OTDR 10, intensity variation data of the total back-scattered light of the optical fiber 7 which is a measurement target is calculated based on particular wavelength light passing only a wavelength component of one of four-wave mixed light generated by interaction of two wavelengths launched among the total back-scattered light by the optical band pass filter 9.
The intensity variation data of the total back-scattered light calculated by the OTDR 10 is stored in RAM within a PC (personal computer) 11 and is used for various calculations.
A conventional procedure of measurement of wavelength dispersion distribution characteristics of FIG. 2 will be described in detail using FIG. 3.
As shown in FIG. 2, in a state of connecting a measured optical fiber 7 to a measuring apparatus, in the case of starting measurement of dispersion distribution characteristics of the measured optical fiber,
First, measurement conditions of the two laser sources 1, 2 of different wavelengths, the OTDR and the EDFA are set (step S11).
After setting the measurement conditions of step S11, measurement of intensity variation data of total back-scattered light of the measured optical fiber is made by the OTDR (step S12).
The measured data measured in step S12 is given to a personal computer (step S13).
Calculation processing of a wavelength dispersion value etc. is performed by processing the measured data from the OTDR by the personal computer (step S14).
Numerical values and waveforms of the wavelength dispersion value and total dispersion value obtained by the processing of step S14 are displayed on a display part (not shown) (step S15).
However, in measurement of the conventional wavelength dispersion distribution measuring apparatus as described in FIG. 2, measurement of dispersion distribution characteristics was made from one side of the measured optical fiber, so that when the measured optical fiber is long, there was a problem that measurement of dispersion distribution of the far end of the measured optical fiber cannot be made accurately due to shortage of output of optical power emitted to the measured optical fiber.
A problem (object) of the invention is to provide a measuring method and a measuring apparatus capable of making accurate measurement of wavelength dispersion distribution characteristics with respect to a long measured optical fiber.
In order to solve the problem, in a measuring method for combining two input light with different wavelengths and emitting a light pulse signal shaped in a pulse waveform to a measured optical fiber and detecting a wavelength component of one of four-wave mixed light generated by interaction of two wavelengths launched among total back-scattered light from the measured optical fiber by an OTDR (optical time domain reflectometer) and measuring wavelength dispersion distribution characteristics of the measured optical fiber, wavelength dispersion distribution characteristics of the optical fiber are measured by the steps of emitting the light pulse signal from one end of the measured optical fiber and measuring a first wavelength dispersion distribution characteristic of the measured optical fiber, emitting the light pulse signal from the other end of the measured optical fiber and measuring a second wavelength dispersion distribution characteristic of the measured optical fiber, and performing superimposition processing of measured results of the first and second wavelength dispersion distribution characteristics, and thereby measurement in which shortage of output of optical power is compensated can be made even for a long measured optical fiber (aspect 1).
Also, the measured results are displayed by a step of displaying numerical values and waveforms of a wavelength dispersion value and a total dispersion value obtained while performing the superimposition processing by a CPU (aspect 2).
Also, automatic measurement can be made by performing a step of switching changeover switches connected before starting measurement in order to emit the light pulse signal to either of both ends of the measured optical fiber between the step of measuring the first wavelength dispersion distribution characteristic and the step of measuring the second wavelength dispersion distribution characteristic (aspect 3).
Also, a wavelength dispersion distribution measuring apparatus of an optical fiber is constructed of light signal generation section for generating two light signals with different wavelengths, section for combining the two light signals with different wavelengths and shaping the signals into a light pulse signal, directional coupling section for inputting the light pulse signal to one end or the other end of a measured optical fiber and also branching total back-scattered light from the measured optical fiber, wavelength extraction section for passing only a wavelength component of one of four-wave mixed light generated by interaction of two wavelengths launched among the total back-scattered light branched, measuring section for measuring wavelength dispersion distribution data from the wavelength component extracted, changeover section which is provided between the directional coupling section and the measured optical fiber and switches the input side and the terminal side of the light pulse from one end to the other end of the measured optical fiber, and calculation section for performing superimposition processing of two measured results associated with switching by the changeover section (aspect 4).
Also, the changeover section comprises first and second changeover switches for selectively performing switching to the side connected to both of the end sides of the measured optical fiber respectively and the side connected to the terminator side respectively, and a third changeover switch for selectively switching the measured optical fiber side of the first and second changeover switches to the input side of the light pulse signal (aspect 5).